[DMN Designer] Exception in read only session

There occurs a JavaScriptException if user enters a read only session during an expression editor is shown.

Manual Acceptance test

1. Prepare 3 files, each having multiple versions stored
2. Open first, do not changes
3. Open second (don't close first by close button, just hide), do some changes do not close
4. Open third (don't close first and second by close button, just hide), do changes and save, swith to older version, do not restore
5. Switch to second file, select older version, restore
6. Switch to first, select older version, do not restore, select again the latest